Multidrug efflux transporters recognize and export various toxic compounds, thereby conferring antibiotics resistance to the multidrug-resistance pathogens. Understanding of this substrate recognition and export mechanism by multidrug exporters is important from the point of the view of drug discovery. However, the structural analysis of membrane transporters is very difficult. As a result of recent advance of the structural analysis, several crystal structures of multidrug efflux transporters have been determined, but their resolutions were not enough to understand the biologically important aspects, such as the substrate recognition mechanism. In this research project, we performed the structural and functional analyses of YddG, which was initially identified as an efflux pump of toxic compounds, and then shown to export aromatic amino acids by subsequent researches.